Handover control method and system

ABSTRACT

In handover control methods and systems which switch a radio base station serving as a communicating counterpart of a mobile station, the present invention provides a handover control method and a system thereof wherein a detection is made when a communication while a predetermined minimum bandwidth secured is not possible at any mobile station communicating with the radio base station and upon the detection the communicating counterpart of the mobile station that communicates with the radio base station mentioned above is switched from the radio base station to other radio base stations.

TECHNICAL FIELD

[0001] The present invention relates to a control system that controls amobile station in mobile communications, typically cellular phonecommunications, and specifically relates to the control of a mobilestation when it switches base stations that the mobile station iscommunicating with, i.e., relates to the system which performs handover.

BACKGROUND TECHNOLOGY

[0002] In the mobile communications, such as cellular phonecommunications, the control of switching of radio base stations tocommunicate with in response to the movement of a mobile station, i.e.,handover, is indispensable. FIG. 21 shows a conventional system thatrealizes handover. The system includes a mobile station 30, two or moreradio base stations (here, a case with two radio base stations is shown)21 and 22, and an RNC (Radio Network Controller: radio network controlunit) 10. The RNC 10 includes an electric field intensity measurementcontrol unit 11 and the radio resources management unit 12 and a switch13.

[0003] Next, FIG. 22 shows the flow of the handover operation. Themobile station 30 is connected to a backbone network through the radiobase station 1 (21) and the switch 13, and communicates through thisbackbone network with terminals that belong to the Internet, the PSTN(Public Switched Telephone Network) and the ISDN (Integrated ServiceDigital Network) which are operated by communication carriers other thanthe mobile communication carrier that manages the backbone network. Whenthe mobile station 30 moves away from the radio base station 1 (21) andapproaches the radio base station 2 (22), the electric field intensityfrom the radio base station 1 (21) becomes weaker, and, conversely, theelectric field intensity from the radio base station 2 (22) becomesstronger. Since the electric field intensity measurement control unit 11requires the mobile station 30 to measure and report the electric fieldintensities of nearby radio base stations including the radio basestation 1 (21) (S1 of the operation flow), the mobile station 30measures the electric field intensities of two or more radio basestations around itself, and transmits the information to the electricfield intensity measurement control unit 11 through the radio basestation 1 (21) on the communication line (S2 and S3 of the operationflow). The electric field intensity measurement control unit 11 informsthe radio resources management unit 12 that handover will take place,when the electric field intensity of the radio base station 2 (22)becomes higher than the electric field intensity of the radio basestation 1 (21) (S4 of the operation flow).

[0004] The radio resource management unit 12 controls radio resourcesrequired for communication between the mobile station and the radio basestation. So to speak, radio resources are physical parameters, types ofwhich are dependent on multiple accessing methods and duplexing methodsthat are used by a radio system. Specifically, in FDMA (FrequencyDivision Multiple Access), it is the bandwidth of a carrier wave, andthe appropriated time length of the carrier wave to a certaincommunication, and, in TDMA (Time Division Multiple Access), it is thebandwidth of the carrier wave, and a number of time slots that areslices in a fixed interval of an appropriated time of the carrier wave.Further, in the case of CDMA (Code Division Multiple Access), it is thebandwidth, and transmission electric power density of the carrier wave,which is dependent on the spread factor. The maximum values of thesevalues are determined by the maximum transmission capacity of each radiobase station and a mobile station.

[0005] The radio resource management unit which manages radio resourcessuch as above assigns the amount of radio resources equivalent to theradio resources which were being assigned to the radio base station 1(21) to the radio base station 2 (22), in order to communicate with themobile station 30, and sets up a new radio channel (S5 of the operationflow). That is, even if the communicating counterpart of the mobilestation 30 is switched from the radio base station 1 (21) to the radiobase station 2 (22), the transmission capacity of the mobile station 30and the radio base station does not change. However, when an amount ofthe radio resources equivalent to prior to the switching was notavailable (S6 of the operation flow), or the electric field intensity ofthe radio channel newly set up was insufficient and not suitable forcommunication (S8 of the operation flow), re-selection of radioresources, or handover is suspended (S11 of the operation flow).Simultaneously, the radio resources management unit 12 controls theswitch 13, and switches the connection between the backbone and theradio base station 1 (21) to the connection between the backbone and theradio base station 2 (22) (S9 of the operation flow). Further, the radioresources management unit 12 controls the mobile station 30 through theradio base station 1 (21) so as to switch the communicating counterpartfrom the radio base station 1 (21) to the radio base station 2 (22) (S10of the operation flow). Since the switching control is performedsynchronously, there is almost no communication interruption during thehandover.

[0006] The control procedure mentioned above is called Mobile-AssistedHandover and used by PDC (Personal Digital Cellular TelecommunicationSystem). Explanation of this procedure is detailed in “Personal DigitalCellular Telecommunication System ARIB Standard RCR STD-27H edition”,published by Association of Radio Industries and Businesses.

[0007] As mentioned above, in the conventional handover system, thehandover is triggered by monitoring and checking the electric fieldintensity or the amount of radio wave interference. For this reason, inconventional systems, the handover is not triggered when an amount oftraffic which goes via a specific radio base station increases becauseof increase in the user's data volume of the mobile station 30. For thisreason, a problem occurs that the radio resources of the radio basestation run short, and the available communication bandwidth decreases.Although the communication itself can be secured in such a system thatoffers the packet telecommunication service which communicates userinformation in packets only when the user data to be transmitted isgenerated, for example, PDC-P (PDC mobile packet communications Systems)even if the radio resources run short, a sharp fall in a throughput iscaused and there is a possibility of producing an inconvenience to theuser. Further, it has been difficult to efficiently utilize radioresources of a mobile communications network as a whole by making amobile station to communicate with another radio base station that has amargin in the radio resources, when the shortage of radio resources of acertain radio base station takes place.

[0008] Further, a minimum bandwidth guarantee type IP (InternetProtocol) communication service which guarantees only the minimum of athroughput is currently offered as a cheap communication service in awired communications network. However, in the conventional mobilecommunications, since there was a possibility of causing the sharp fallin the throughput by the increase in traffic as mentioned above, it wasdifficult to apply such bandwidth guarantee service to mobilecommunications.

[0009] Further, conventionally, handover systems have been proposed (forexample, JP, 2000-175243) to cope with a traffic congestion condition inthe communication of a radio base station by switching a mobile stationfrom the station with which the communication has been made to anadjacent radio base station. In such a handover system, directions ofhandover are made to all the mobile stations that communicate with theradio base station which is experiencing the traffic congestioncondition. In addition, the handover takes place to each adjacent basestation from all the mobile stations that are connected to the radiobase station in the traffic congestion condition and that are capable ofcommunicating with adjacent radio base stations.

[0010] If all the mobile stations are switched to the adjacent radiobase stations from the radio base station that is in a trafficcongestion condition in this way, the traffic amount in the congestedradio base station will be lowered. However, depending upon trafficsituation of the adjacent radio base stations to which the handover hasbeen made, some of handed-over mobile stations may experience such asituation that it would have had a wider bandwidth available if itstayed with the original radio base station. In this case, the qualityof the communication service which can be received by the mobile stationwill deteriorate.

DISCLOSURE OF INVENTION

[0011] The general purpose of the present invention is to offer a methodand a system of a new and useful handover control, solving the problemof the conventional technology mentioned above.

[0012] The specific purpose of the present invention is to offer themethod and the system of the handover control which enable the handoverof the mobile station that communicates with the radio base station in acondition of traffic congestion to secure a level of communicationservice quality desired by a user as much as possible, when the radiobase station experiences the traffic congestion.

[0013] The purpose of the present invention is served by the handovercontrol method that detects an inability to communicate with apredetermined minimum bandwidth secured of any mobile stations in motionthat are communicating with the radio base station, and switches thecommunicating counterpart of the mobile station that communicates withthe radio base station mentioned above from the radio base station toanother radio base station among handover control methods that switch aradio base station serving as the communicating counterpart of a mobilestation.

[0014] In the handover control method, the communicating counterpart ofthe mobile station that communicates with the base station is switchedto another radio base station when a communication securing thepredetermined minimum bandwidth becomes impossible at any mobilestations in communication with the radio base station.

[0015] The mobile station whose radio base station serving as thecommunicating counterpart is to be switched may be the mobile stationthat has become incapable of communicating under a condition thatsecures the above-mentioned minimum bandwidth or any of other mobilestations. Further, the number of the mobile stations whose radio basestation serving as the communicating counterpart is to be switched maybe one or more.

[0016] Depending on the amount of radio resources currently used in theradio base station after the switching, when the communicatingcounterpart of the mobile station that experiences an impossibility ofcommunication securing the minimum bandwidth mentioned above is switchedto the other radio base stations, the communication securing the minimumbandwidth mentioned above may become possible. Further, when thecommunicating counterpart of mobile stations other than the mobilestation communication that became impossible to communicate securing theabove-mentioned minimum bandwidth are switched to other radio basestations, the radio resources used by the other mobile station can beassigned to communication of the mobile station communication that hasbecome impossible to communicate, enabling a communication with theminimum bandwidth secured.

[0017] The purpose of the present invention is served by the handovercontrol method that detects an inability to communicate securing thepredetermined minimum bandwidth by the radio base station and any mobilestations in motion, and switches the communicating counterpart of themobile station that communicates with the radio base station mentionedabove from the radio base station to a plurality of other radio basestations among handover control methods that switch the radio basestation serving as the communicating counterpart of a mobile station.

[0018] In the handover control method, the communicating counterpart ofmobile stations is switched to the plurality of radio base stations.Consequently, radio resources required for communication in a mobilestation is shared by the plurality of the radio base stations.

[0019] The plurality of the radio base stations switched ascommunicating counterparts of the mobile stations which communicate withthe above-mentioned radio base station may also include theabove-mentioned radio base station which was communicating with themobile station concerned, before switching.

[0020] From a view point that it is desirable to switch thecommunicating counterpart of a mobile station to a radio base stationwhose electric field intensity becomes higher, a structure can be suchthat a radio base station that provides the highest electric fieldintensity and the mobile station that receives the signal, excepting thecombination of the mobile station and the radio base station that arecurrently communicating, are selected as the mobile station and theradio base station for the communicating counterpart as the object ofthe handover, based on the information relative to the electric fieldintensity of the radio base station acquired from each mobile station ineach handover control method mentioned above.

[0021] From a view point that it is desirable that the mobile stationwhose communication bandwidth is below a standard is switched as much aspossible to the radio base station which has a sufficient radio resourceavailable, a structure can be such that a mobile station that has theradio resources allocation the closest to the minimum bandwidth, havingno allowance to increase in the user data volume, and a radio basestation that has the largest amount of the radio resources available areselected as the mobile station and the radio base station for thehandover.

[0022] From a similar perspective, a structure can be such that a mobilestation whose minimum bandwidth is the largest and that requires theradio resources, and a radio base station that has the largest amount ofthe radio resources available are selected as the mobile station and theradio base station for the handover.

[0023] From a viewpoint of offering a specific technique for detectingan inability to communicate with the minimum bandwidth secured, thedetection of the inability to communicate with the minimum bandwidthsecured between mobile stations and the base station can be performedbased on measuring results by measuring one or a combination of theelectric field intensity, a bit error rate, a frame error rate, and apacket discard rate.

[0024] Further, the mobile station mentioned above can choose the radiobase station which serves as a communicating counterpart after switchingmentioned above. The radio base station which serves as thecommunicating counterpart after the above-mentioned switching may alsobe selected in a node other than the above-mentioned mobile station, theinformation relative to the selected radio base station is notified tothe mobile station concerned from the node concerned, and the radio basestation which serves as a communicating counterpart of the mobilestation concerned can be switched to the notified radio base station.

[0025] From a viewpoint of enabling handover between radio communicationsystems which adopt different protocols, a structure can be as follows.In the handover control method mentioned above, when it is detected thata communication with the radio base station becomes impossible with theminimum bandwidth secured at any of mobile stations in the first radiocommunications system that adopts the first communication protocol, andupon switching the communicating counterpart of the mobile station thatcommunicates with the radio base station mentioned above to a radio basestation in the second radio communication system that adopts the secondcommunication protocol, the information relative to the radio resourcesof the mobile station concerned is transferred from the first radiocommunication system mentioned above to the second radio communicationsystem mentioned above via a wired section after converting theprotocol.

[0026] Further, the purpose mentioned above relative to the handovercontrol method for switching a radio base station that is thecommunicating counterpart of a mobile station, is served as follows. Acheck is made as to whether or not the communication of the radio basestation will become congested, and if a decision is made that thecommunication of the radio base station will be congested, a choice willbe made from combinations of any mobile stations that communicate withthe radio base station concerned and one or more radio base stationsthat can communicate with the mobile stations concerned according to apredetermined standard, and the communicating counterparts of the mobilestations in the selected combination are switched to one or more radiobase stations.

[0027] In such a handover control method, if a decision is made that thecommunication of a radio base station is in a traffic congestioncondition, a combination of any mobile stations that communicate withthe base station concerned and one or more radio base stations that cancommunicate with the mobile stations is selected in accordance with apredetermined standard In addition, the communicating counterpart of themobile stations in the selected combination is switched to one or moreradio base stations in the combination.

[0028] The number of combinations of the mobile station and one or moreradio base stations to be selected can be singular or plural. Furtherthe mobile station concerned and the above-mentioned radio base stationwhich performs the on-going communication can also be included in theplurality of the radio base stations.

[0029] The traffic congestion condition in the above-mentioned radiobase station can be determined based on various information such as anavailable amount of the radio resources of the radio base stationconcerned, radio resources amount that the mobile station that willcommunicate with the radio base station concerned will use, an errorrate in the communication, and whether or not the communication is beingperformed with the predetermined minimum bandwidth.

[0030] From a viewpoint that the communicating counterpart of a mobilestation can be switched to the radio base station that has a margin inthe radio resources as much as possible, a structure can be such thatthe predetermined standard for selecting the combination of theabove-mentioned mobile station and a radio base station is defined bythe available amount of the radio resources of the radio base station orthe required amount of the radio resources.

[0031] From a viewpoint that the communicating counterpart of a mobilestation can be switched to a radio base station with a bettercommunication condition as much as possible, the predetermined standardfor selecting the combination of the above-mentioned mobile station anda radio base station can be defined based on the receiving electricfield intensity in communication between a mobile station and a radiobase station.

[0032] From a viewpoint that the communicating counterpart of the mobilestation whose required data communication volume is not satisfied can beswitched as often as possible, the predetermined standard for selectingthe combination of the above-mentioned mobile station and a radio basestation can be defined based on a ratio of the amount of radio resourcespermitted to the mobile station to the amount of radio resourcescurrently used in fact.

[0033] From a viewpoint that the switching of the communicatingcounterpart of a mobile station is easy, the predetermined standard forselecting the combination of the above-mentioned mobile station and aradio base station can be defined based on the number of the radio basestations with which a mobile station should perform simultaneouscommunications after switching the communicating counterpart.

[0034] In such a handover control method, the communicating counterpartof a mobile station can be switched with a priority to a fewer number ofradio base stations, for example. In this case, the switching to a fewernumber of radio base stations can be performed comparatively easily.

[0035] From a viewpoint that the radio base station handling theon-going communication can be included in the radio base station towhich the switching is to be performed as much as possible as acommunicating counterpart for the mobile station after the switching,the predetermined standard for selecting the combination of theabove-mentioned mobile station and a radio base station can be definedbased on whether or not to include the radio base station handling theon-going communication as the radio base station to communicate afterthe switching of the communicating counterpart of the mobile station.

[0036] In the above-mentioned handover control method, a structure canbe such that a priority is attached to the combinations of each mobilestation that communicates with the radio base station concerned and oneor more of the radio base station concerned and its adjacent radio basestations, thereby selecting a combination of a mobile station and aradio base station that has a higher priority.

[0037] The above-mentioned priority is the ranking about theappropriateness of a mobile station and a radio base stationcommunicating mutually, and it can be, but is not limited to be,expressed by a mark given in accordance with the predetermined standardor it may be expressed by the ranking itself. That is, the combinationof the mobile station and radio base station where a priority is thehigher means that it is the more suitable for communicating. “Suitablefor communicating” means that a better quality communication can becarried out under a satisfactory condition, and that the controlrequired for handover can be performed easily.

[0038] From a viewpoint that a handover process can be performedcentrally, a structure can be such that, relative to the handovercontrol method mentioned above, the process for selecting a combinationof any mobile station that communicates with the radio base stationconcerned and one or more radio base stations with which the mobilestation concerned can communicate is performed in a node connected witheach radio base station via a wired section.

[0039] From a viewpoint that a process about handover can be performedin a distributed fashion, a structure can be such that, relative to thehandover control method mentioned above, the mobile stationcommunicating with the radio base station concerned performs the processin which it chooses a combination of the mobile station concerned andone or more radio base stations with which a communication is possiblein accordance with the above-mentioned predetermined standard.

[0040] Further, in the above-mentioned handover control method, astructure can be such that the above-mentioned mobile station whichcommunicates with the radio base station concerned determines whetherthe radio base station concerned is in a traffic congestion condition.

[0041] From a viewpoint that the communication of a mobile station ismaintained reliably in high quality, the handover control methodmentioned above can be structured such that when the communicationcondition between the mobile station and one or more radio base stationsis supervised and the supervised communication condition is in acondition worse than a predetermined standard condition, after switchingthe communicating counterpart of the above-mentioned mobile station tothe one or more radio base stations mentioned above, switching of thecommunicating counterpart of the mobile station concerned may beperformed once again.

[0042] In such a handover control method, when the communicationcondition between the mobile station and the radio base station afterthe switching is worse than a predetermined condition after switchingthe communicating counterpart of a mobile station, the communicatingcounterpart of the mobile station is switched again. In this manner, themobile station concerned can ensure communication in the bettercondition.

[0043] In the above-mentioned handover control method, a structure canbe such that an error rate of the communication between a mobile stationand one or more radio base stations is supervised as the communicationcondition mentioned above.

[0044] Further, the above-mentioned purpose in the handover controlsystem that switches a radio base station serving as the communicatingcounterpart of a mobile station is served by a detection means fordetecting that the communication with the predetermined minimumbandwidth secured becomes impossible at any of mobile stationscommunicating with the radio base station, and a switching means forswitching the communicating counterpart of the mobile stationcommunicating with the above-mentioned radio base station from the radiobase station concerned to another radio base station when the detectionmeans detects that any mobile station becomes incapable of communicatingwith the predetermined minimum bandwidth secured.

[0045] Further, the purpose of the present invention described above inthe handover control system that switches a radio base station servingas the communicating counterpart of a mobile station is served by adetection means for detecting that the communication with thepredetermined minimum bandwidth secured becomes impossible at any ofmobile stations communicating with the radio base station, and aswitching means for switching the communicating counterpart of themobile station communicating with the above-mentioned radio base stationfrom the radio base station concerned to a plurality of other radio basestations when the detection means detects that any mobile stationbecomes incapable of communicating with the predetermined minimumbandwidth secured.

[0046] Further, the purpose of the present invention described above inthe handover control system that switches a radio base station servingas the communicating counterpart of a mobile station is served by ahandover system that includes a traffic congestion checking means forchecking whether or not the communication traffic in the radio basestation becomes congested, a selection means for selecting a combinationof a mobile station communicating with the radio base station concernedand one or more radio base stations that are capable of communicatingwith the mobile station concerned according to a predetermined standardwhen the traffic congestion checking means determines that thecommunication traffic of the radio base station is congested, and aswitching control means for switching the communicating counterpart ofthe mobile station in the combination selected to one or more radio basestations in the combinations that are selected.

[0047] In addition, other purposes, features, and advantages of thepresent invention are clarified by detailed explanation in the followingwith reference to appended drawings.

BRIEF EXPLANATION OF THE DRAWINGS

[0048]FIG. 1 is a block diagram showing the first example of thehandover system of the present invention.

[0049]FIG. 2 shows the operation flow of the handover system of thepresent invention.

[0050]FIG. 3 is a block diagram showing the second example of thehandover system of the present invention.

[0051]FIG. 4 shows an example of structure of a signalsplitting/combining unit of the present invention.

[0052]FIG. 5 shows the operation flow of the second example of thepresent invention.

[0053]FIG. 6 is a block diagram showing the third example of thehandover system of the present invention.

[0054]FIG. 7 shows the operation flow of the third example of thepresent invention.

[0055]FIG. 8 is a block diagram showing the fourth example of thehandover system of the present invention.

[0056]FIG. 9 shows an example of structure of a mobile communicationssystem to which the fifth example of the handover system of the presentinvention is applied.

[0057]FIG. 10 is a block diagram showing the fifth example of thehandover system of the present invention.

[0058]FIG. 11 is a sequence drawing showing an example of the procedureof a handover process.

[0059]FIG. 12 is a sequence drawing showing an example of the processprocedure for selecting the optimum mobile station and a radio basestation.

[0060]FIG. 13 shows an example (No. 1) of an evaluation point table.

[0061]FIG. 14 shows an example (No. 2) of an evaluation point table.

[0062]FIG. 15 shows an example (No. 3) of an evaluation point table.

[0063]FIG. 16 shows an example of a management table for an evaluationpoint.

[0064]FIG. 17 is a sequence drawing showing an example of the processprocedure for selecting the optimum mobile station and a radio basestation in the sixth example of the handover system of the presentinvention.

[0065]FIG. 18 is a flowchart which shows an example of the processprocedure for selecting the optimum mobile station and a radio basestation in the seventh example of the handover system of the presentinvention.

[0066]FIG. 19 is a block diagram showing the eighth example of thehandover system of the present invention.

[0067]FIG. 20 is a sequence drawing showing an example of the procedureof handover process.

[0068]FIG. 21 is a block diagram showing a conventional example.

[0069]FIG. 22 shows the operation flow of the conventional example.

THE BEST MODE IMPLEMENTATION OF THE INVENTION

[0070] Hereafter, embodiments of the present invention will be describedbased on the drawings.

[0071]FIG. 1 is the block diagram showing the first example of thehandover system concerning the implementation mode of the presentinvention.

[0072] This system includes mobile stations (here 1 and 2) 31 and 32,two or more radio base stations (here 1 and 2) 21 and 22, and an RNC 10.The RNC 10 includes a minimum bandwidth guarantee unit 14, an electricfield intensity measurement control unit 11, radio resources managementunit 12, and a switch 13.

[0073]FIG. 2 shows the operation flow of this system. First, the mobilestation 1 (31) and the mobile station 2 (32) shall communicate withdifferent terminals in the Internet, PSTN and ISDN through the radiobase station 1 (21), the switch 13, and a backbone. A description willbe made as to the case where data volume of the user of the mobilestation 1 (31) increases, and the radio base station 1 (22) is becomingin a traffic congestion condition at this time. First, the minimumbandwidth guarantee unit 14 stores the minimum bandwidth required foreach user and for each communication service beforehand (operation flowS1). Next, the minimum bandwidth guarantee unit 14 sets up the minimumvalue of the amount of radio resources which satisfies the minimumbandwidth of the user request based on this information (S2 of theoperation flow). For example, the minimum required numbers of radiocarriers, slots, or spreading codes, either independently or anycombination thereof are set up. In addition, it is also possible torestrict the number of the mobile stations that make a random access toa specific radio channel.

[0074] The radio base station 1 (21) measures the amount of trafficwhich passes through itself, and if it is congested, it will notify thefact to the radio resources management unit 12. The radio resourcemanagement unit 12 needs to allocate additional radio resourcecorresponding to the increase in traffic in the radio base station.However, the radio resources of one radio base station are limited, andruns short of the radio resources. Then, the radio resources managementunit 12 inquires the minimum bandwidth guarantee unit 14 whethercommunication of the radio base station 1 (21) with each mobile stationis satisfying the minimum bandwidth (S3 of the operation flow). Upon theinquiry (S4 of the operation flow), if there is a mobile station forwhich the minimum bandwidth is not satisfied, the radio resourcesmanagement unit 12 will select a mobile station (the mobile station 1(31) in this description) and a radio base station (the radio basestation 2 (22) in this description) for carrying out the handover. Asselection methods,

[0075] (1) a method to choose a radio station that gives the highestelectric field intensity, and the mobile station which received this,based on the electric field intensity information on the radio basestation obtained from each mobile station through the electric fieldintensity measurement control unit 11,

[0076] (2) a method to choose a mobile station with an allocation ofradio resources the closest to the minimum bandwidth, and a radio basestation in which the radio resources is left the most based on the radioresources management information,

[0077] (3) a method to choose a mobile station whose value of theminimum bandwidth is large and needs a large amount of the radioresources, and a radio base station that has the largest amount of theradio resources available, and the like are possible. Further, a set ofthe plurality of the methods may be used. Further, radio resources isassigned to the radio base station 2 (22) so that the radio resources ofwhich amount may not be less than the minimum value of the amount ofradio resources defined by the minimum bandwidth guarantee unit 14 tothis radio base station, setting up a new radio channel (S6 of theoperation flow). Next, the electric field intensity measurement controlunit 11 directs the mobile station 1 (31) to supervise whether the newlyset-up radio channel is suitable for communication via the radio basestation 1 (21) (S7 of the operation flow). If suitable forcommunication, the electric field intensity measurement control unit 11directs execution of the handover to the radio resources management unit12. The radio resource management unit 12 controls the switch 13, andswitches the connection between the backbone and the radio base station1 (21) to the connection between the backbone and the radio base station2 (22) (S9 of the operation flow). Further, the radio resourcesmanagement unit 12 controls the radio base station 1 (21), the radiobase station 2 (22), and the mobile station 1 (31) to switch thecommunicating party of the communication of the mobile station from theradio base station 1 (21) to the radio base station 2 (22) completely(S10 of a work flow). These switching actions are performedsynchronously.

[0078] In addition, this handover system is also equipped with functionsto supervise a fall in the electric field intensity, an occurrence ofradio wave interference and an occurrence of fading in performing thehandover as in conventional systems.

[0079]FIG. 3 shows the second example of the handover system concerningthe implementation mode of the present invention. In this system, whenneither the radio base station 1(21) nor the radio base station 2 (22)can singly provide a sufficient radio resource, the minimum bandwidth issecured by communicating with both the radio base station 1 (21) and theradio base station 2 (22) by adding up the data transmission capacity ofboth communications. When it still runs short, the third and fourthradio base stations will be selected, and the minimum bandwidth isguaranteed by communicating with a number of radio base stations. Thesystem of the present invention includes the mobile stations (here 1 and2) 31 and 32, two or more radio base stations (here 1 and 2) 21 and 22,and the RNC 10. The RNC 10 includes the minimum bandwidth guarantee unit14, the signal splitting/combining unit 15, the electric field intensitymeasurement control unit 11, the radio resources management unit 12, andthe switch 13. The example of the signal splitting/combining unit 15 isshown in FIG. 4. In the unit, circuits are divided into the upstreamcircuit and the downstream circuit. Signals are input to the upstreamcircuit from the base station and to the downstream circuit from anexchange. These signals are divided into signals for every radio basestation in the case of the downstream signal and for every mobilestation in the case of the upstream signal by the distribution units151, 152, 161, and 162. The distributed signal is compounded andoutputted by the combining units 153, 154, 163, and 164 according to thesequence of transmission at the transmitting source.

[0080] On the other hand, the mobile station compliant with this systemis equipped with two or more transceiver units for communicating withtwo or more radio base stations, and signal splitting/combining unitsfor distributing user information to two or more transceiver units. Inaddition, the structure of the signal splitting/combining unit may beobtained by eliminating the combiner in the upstream circuit and thesplitter in the downstream circuit, and further using one input terminalon the upstream side and one output terminal on the downstream side inreference with FIG. 4 presented above.

[0081]FIG. 5 shows the operation flow. When it is determined that theminimum bandwidth desired by a user is not satisfied (S4 of flow ofoperation), the radio resources management unit 12 chooses a pluralityof mobile stations and radio base stations for which handover is to takeplace (S5 of the operation flow). As specific selection methods, themethod of giving priority to combinations where a high electric fieldintensity of the radio base station is measured by the mobile stations,the method of giving priority to combinations that leave more room ofthe radio resources, and the method that combines these methods arepossible. Next, the radio resources management unit 12 assigns radioresources so that the minimum bandwidth which the mobile station 1 (31)needs may be satisfied, and a new radio channel may be set up relativeto the selected mobile station 1 (31) and selected two or more radiobase station (S6 of the operation flow). Next, the electric fieldintensity measurement control unit 11 directs the mobile station 1 (31)to measure the electric field intensity and report the result in orderto determine whether the radio channel which the radio resourcesmanagement unit 12 newly set up is suitable for communication. If theelectric field intensity measurement unit 12 determines that the newlyset up channel is suitable for communication, it directs the radioresources management unit 12 to carry out handover. The radio resourcemanagement unit 12 controls the splitting/combining unit 15, splittingand combining signals for each mobile station (S9 of the operationflow). Further, the radio resources management unit 12 controls theswitch 13, switching the signal for every mobile station, and sends outthis signal to the Internet or the like through the backbone (S10 of theoperation flow).

[0082]FIG. 6 shows the third example which added the bandwidth measuringunit 16 that supervises whether the minimum bandwidth of each user andeach communication service is secured to the handover system mentionedabove. FIG. 7 shows the operation flow. In the system of the presentinvention, the bandwidth measuring unit 16 may be installed, forexample, between the switch 13 and the backbone to measure throughput ofeach user and all or any of the communication services (S12 of theoperation flow). Specifically, as for the information signal which has aframe structure, each frame has an error detection function. For thisfunction, CRC (Cyclic Redundancy Control) which adds a redundant codemay be used, for example. The throughput in unit time is obtained usingthis facility by disregarding frames if a frame error is detected where,for example, there is no retransmission performed. Further, when themeasurement result shows non-fulfillment of the minimum bandwidth (S13of the operation flow), the minimum bandwidth guarantee unit 14re-assigns radio resources and a new radio channel is set up.

[0083]FIG. 8 shows the fourth example where a handover system is appliedto a handover between systems that use different communication protocolsfor the radio transmission path. Structure of this system includes asystem 2 that has a similar structure to conventional examples andperforms equivalent operations, and a system 1 that has the system 2under itself and that has the radio transmission path of a differentcommunication protocol from the system 2. As an example of this case,the system 1 may correspond to IMT-2000 and the system 2 may correspondto PDC.

[0084] The system 1 includes the mobile stations (here 1 and 2) 31 and32, two or more radio base stations (here 1 and 2) 21 and 22, and theRNC 10. The RNC 10 includes the minimum bandwidth guarantee unit 14,protocol converters 17 and 18, the electric field intensity measurementcontrol unit 11, the radio resources management unit 12, and the switch13.

[0085] In this system, the switch 13 of the system 1 and a switch 103 ofthe system 2 are connected through the protocol converter 17. Further,the minimum bandwidth guarantee unit 14 of the system 1 and the radioresources management unit 102 of the system 2 are also connected throughthe protocol converter 18. Here, the communication protocols of theradio transmission path in this system differ in the system 1 and thesystem 2. Therefore, in order that a mobile station complies with thecommunication protocol of both systems, it is equipped with atransceiver unit for the system 1, and a transceiver unit for the system2. Further, it also has a switch that switches the two transceiver unitsby control from the system 1 or the system 2.

[0086] Although operation of this system is almost equal to the contentsof the operation described in the first example mentioned above, itdiffers in that the minimum bandwidth guarantee unit 14 communicateswith the radio resources management unit 102 of a system 2 via theprotocol converter 18. Further, the radio resources management unit 12of the system 1 and the radio resources management unit 102 of thesystem 2 exchange information with each other via the protocol converter18 when adjusting between the two for assignment of radio resources.

[0087] Further, the fifth example of the handover system concerning theimplementation mode of the present invention will be described. In thisfifth example, when a radio base station experiences a trafficcongestion condition, mobile stations suitable for handover to adjacentbase stations will be selected from mobile stations which arecommunicating with the radio station, and the selected mobile stationsare handed-over to the adjacent radio base stations.

[0088] The mobile communications system to which the handover systemrelative to this fifth example is applied is structured as shown in FIG.9.

[0089] In FIG. 9, radio base stations BS1, BS2, and BS3 are connected tothe RNC 10, and the RNC 10 is connected to the backbone. By suchstructure, each of the mobile stations MS1 through MS5 which are locatedin the communication area of each of the radio base stations BS1, BS2,and BS3 can communicate with terminals in PSTN (Public SwitchedTelephone Network), ISDN (Integrated Service Digital Network), theInternet, and the like through the radio base stations BS1, BS2, BS3,the RNC 10 and the backbone. While this fifth example illustrates thestructure with five mobile stations MS1 through MS5 communicating withthe three radio base stations BS1, BS2, and BS3, this is an example andis not specifically limited to it.

[0090] The handover system as mentioned above applied to the mobilecommunications systems is structured as shown in FIG. 10.

[0091] In FIG. 10, the RNC 10 has the electric field intensitymeasurement control unit 11, the radio resources management unit 12, andthe switch 13 like the example mentioned above. Further, the RNC 10 isequipped with mobile station management unit 19. The mobile stationmanagement unit 19 evaluates combinations of each mobile station andeach radio base station which should communicate with each of the mobilestations, using an evaluation point table that is predefined fromviewpoints which are mentioned later, such as receiving electric fieldintensity and required radio resource. In addition, according to theevaluation result, an optimum combination of a mobile station and aradio base station for the handover is selected from the combinations ofeach mobile station and radio base stations.

[0092] A description will follow as regards the handover performed, forexample, when the mobile station MS1 which communicates with the radiobase station BS1 moves to the communication area of the radio basestation BS2 from the communication area of the radio base station BS1,with reference to FIGS. 9 and 10.

[0093] As shown in FIG. 9, the radio base station BS1 communicates witheach of the mobile stations MS1 and MS5 which are currently in itscommunication area, and the radio base station BS2 is communicating witheach of the mobile stations MS2, MS3, and MS4 which are currently in itscommunication area. In specific reference to the mobile station MS1 andthe mobile station MS2, the communication sequence is as shown in FIG.11, for example.

[0094] As shown in FIG. 11(a), where the mobile station MS1 communicateswith the radio base station BS1, and the mobile station MS2 communicateswith the radio base station BS2, measurement directions for thereceiving electric field intensity are transmitted from the electricfield intensity measurement control unit 11 in the RNC 10 to each of themobile stations MS1 and MS2 through each of the radio base stations BS1and BS2. According to the directions, each of the mobile stations MS1and MS2 measures the electric field intensity from each of radio basestations which are currently in the vicinity, and report the measuredvalues to the RNC 10 through the radio base stations BS1 and BS2.

[0095] The mobile station MS1 communicating with the radio base stationBS1 moves to the communication area of the radio base station BS2. Inthat case, as shown in FIG. 11(b), the measured value of the electricfield intensity of the radio base station BS1 and the measured valued ofthe electric field intensity of the radio base station BS2 which arereported to the electric field intensity measurement control unit 11 ofthe RNC 10 from the mobile station MS1 are compared. And if the electricfield intensity from the radio base station BS2 is higher than theelectric field intensity from the radio base station BS1 by apredetermined amount, it triggers the RNC 10 to start the control forswitching the communicating counterpart of the mobile station MS1 fromthe radio base station BS1 to the radio base station BS2 (a handover)(handover 1). However, the situation of the radio base station BS2 isthat that a large amount of its radio resource is being used in thiscase by the communications with the mobile stations MS2, MS3, and MS4.If the radio base station BS2 determines that it will run short of theradio resources for newly assigning communication with the mobilestation MS1 (traffic will be congested), a direction is made from theradio base station BS2 to the radio base station BS1 through the RNC 10to suspend the handover of the mobile station MS1, and the direction isfurther transmitted to the mobile station MS1 from the radio basestation BS1.

[0096] Thus, the mobile station MS1 which received the direction tosuspend the handover maintains the communication condition with theradio base station BS1. As shown in FIG. 11(c), in this condition, theradio base station BS2 reports to the RNC 10 that it is short of theradio resources. The RNC 10 which received this report performs processfor selecting mobile stations which are suitable for handover to theadjacent radio base stations of each of the mobile stations MS2, MS3,and MS4 which are communicating with the radio base station BS2 in orderto enable communication between the mobile station MS1 moving into thecommunication area of the radio base station BS2, and the radio basestation BS2. Details of this process will be described later.

[0097] When a combination of a mobile station which should carry outhandover and a radio base station to which the handover should becarried out is selected, the radio base station in the combination ismade to carry out the handover of the mobile station in the selectedcombination (handover 2). For example, when it is decided that the radiobase stations BS2 and BS3 share the communication of the mobile stationMS2 at which the handover is carried out (refer to the second example),a link is established between the mobile station MS2 and the radio basestations BS2 and BS3 by the handover processes (electric field intensitymeasurement and reporting, base station switching control, and the like)of the mobile station MS2. In addition, the mobile station MS2 performsradio communications with the radio mobile stations BS2 and BS3 inparallel, and further communicates with other communication terminalsthrough the RNC 10 and the backbone.

[0098] Thus, if part of the radio resources which are needed forcommunication of the mobile station MS2 is shared by the radio basestation BS3 (partial handover of the mobile station MS2), the radioresources for the mobile station MS1 moved to the communication area ofthe radio base station BS2 communicating with the radio base station BS2can be secured. In this condition, the RNC 10 will resume the handoverprocess of the mobile station MS1 suspended in the above (refer to (d)in FIG. 12). That is, the RNC 10 directs resumption of the base stationswitching control to each of the radio base stations BS1 and BS2 and themobile station MS1. Consequently, the communicating counterpart of themobile station MS1 is switched from the radio base station BS1 to theradio base station BS2 (handover). Henceforth, the mobile station MS1communicates with other terminals through the radio base stations BS2,the RNC 10 and the backbone.

[0099] Thus, when the mobile station MS1 has moved to the communicationarea of the radio base station BS2, possibly causing a trafficcongestion status at the radio base station BS2, the radio resources forcommunication between the mobile station MS1 and radio base station BS2can be secured by making the adjacent radio base station BS3 carry outthe handover (also including partial handover) of the mobile stationwhich has already communicated with the radio base station BS2. Inaddition, by performing the above-mentioned process for other mobilestations in a chain reaction, reliable communication can be given to asmany mobile stations as possible.

[0100] As mentioned above, in order to enable a communication betweenthe mobile station MS1 moved to the communication area of the radio basestation BS2, and the radio base station BS2 concerned, the selectionprocess is performed to choose a mobile station suitable for handover toanother adjacent radio base station out from the mobile stations MS2,MS3, and MS4 which are already in communication with the radio basestation BS2 (referring to drawing 11 (c)), process of which is madeaccording to the procedure specifically shown in FIG. 12.

[0101] In FIG. 12, the electric field intensity measurement control unit11 of the RNC 10 directs subordinate mobile stations to measure thereceiving electric field intensity of the radio base station with whichthe mobile stations are communicating and which are adjacent to them.And each of the mobile stations MS1 and MS2 reports the measured data ofthe electric field intensity to the RNC 10 via the radio base stationsBS1 and BS2, respectively ((1) in FIG. 12).

[0102] Further, the radio resources management unit 12 of the RNC 10directs each of the radio base stations BS1, BS2, and BS3 to report theusage situation of the radio resources. And each of the radio basestations BS1, BS2, and BS3 reports the usage situation of the radioresources to the radio resources management unit 12 of the RNC 10according to the direction ((2) in FIG. 12).

[0103] As the information indicative of the radio resources usage, aratio of the maximum amount of radio resources permitted for each mobilestation communicating with each radio base station, to the radioresources actually occupied by the mobile station, an occupancy rate ofthe radio resources in each radio base station, and the like can beused, for example.

[0104] When the electric field intensity measurement control unit 11receives the report of the measured value of the electric fieldintensity from each mobile station as mentioned above and the radioresources management unit 12 receives the report of the radio resourcesusage situation from each radio base station, the mobile stationmanagement unit 19 of the RNC 10 will perform evaluation of combinationsof a mobile station and a radio base station serving as a communicatingcounterpart based on the reported measured values of the electric fieldintensity, and the radio resources usage situation. In addition, acombination of a mobile station suitable for carrying out handover basedon the evaluation and a radio base station as its handover destinationis selected ((3) in FIG. 12).

[0105] The result of the selection, that is, the combination of themobile station and the radio base station as the handover destination isnotified to each of the radio base stations BS1, BS2, and BS3 from theRNC 10 ((4) in FIG. 12). Subsequently, the handover is carried out forthe selected mobile station to the radio base station in the combination(refer to FIG. 11(c)).

[0106] Evaluation of the combination mentioned above of the each mobilestation and the radio base station serving as a communicatingcounterpart is made as follows, for example.

[0107] As a communicating counterpart to each mobile station, a radiobase station which gives the measured value of receiving electric fieldintensity as strong as possible is suitable. For a combination of aradio base station BS_(i) and a mobile station MS_(j), if the measuredvalue of electric field intensity is P_(ij), the evaluation point isexpressed as a function of the measured value P_(ij), of the electricfield intensity f(P_(ij)). The evaluation point f(P_(ij)) is definedbased on an evaluation point table as shown in FIG. 13. In the case ofthis example (refer to FIG. 13), it is supposed that the proper dynamicrange of the received electric field in a mobile station is 4 dBμV-64dBμV.

[0108] When the measured value P_(ij) of the received electric fieldfrom the radio base station B_(i) (partner station) which is currentlycommunicating with the mobile station MS_(j) becomes below theabove-mentioned dynamic range, the evaluation point f (P_(ij)) is “zeropoint” (the lowest point), and the evaluation point f (P_(ij)) becomesthe higher one by one like “one point”, “two points”, “three points”,and “four points” as the measured value P_(ij) of the electric fieldintensity becomes the higher. Therefore, it indicates that the higherthe evaluation point is for the combination of the mobile station MS_(j)and the radio base station BS_(i) which is the current communicatingcounterpart, the more desirable that the communication with this radiobase station BS_(i) is maintained for the mobile station MS_(j).

[0109] On the other hand, if the measured value P_(ij) of the receivedelectric field from a radio base station (adjacent station) BS_(i) whichis adjacent to the radio base station which is currently communicatingwith the mobile station MS_(j) becomes below the above-mentioned dynamicrange, like the case of the radio base station (partner station) whichserves as a communicating counterpart mentioned above, theabove-mentioned evaluation point f (P_(ij)) is “zero” (the minimumpoint), and the evaluation point f (P_(ij)) becomes the higher one byone like “one point”, “two points”, “three points”, and “four points” asthe measured value P_(ij) of the electric field intensity becomes thehigher. Therefore, it indicates that the higher the evaluation pointbecomes for the combination of the mobile station MS_(j) and theadjacent station BS_(i), the more desirable becomes the handover of themobile station MS_(j) to take place to the adjacent station BS_(j).

[0110] Namely, it indicates that the higher the evaluation point is fora combination of the mobile station MS_(j) and the radio base stationBS_(i) (whether the partner station or any of adjacent stations), themore it is desirable for the mobile station MS_(j) to communicate withthe radio base station BS_(i) of the combination.

[0111] Further, as a communication counterpart of a mobile base station,a radio base station that can provide as much radio resource as possiblefor the mobile station to occupy is suitable. Therefore, when a ratio ofthe amount of maximum radio resources permitted to the mobile stationMS_(j) and the radio resources actually occupied by communication is setto r_(j) in relation to the combination of the radio base station BS_(i)and the mobile station MS_(j) and a rate of the radio resources usage inthe radio base station BS_(i) is set to R_(i), the correspondingevaluation point for each is expressed by functions g₁ (r_(j)) andg₃(R_(i)), respectively. Based on an evaluation point table such asshown in FIG. 14, each of these evaluation points g₁ (r_(j)) andg₂(R_(i)) is defined.

[0112] When the ratio r_(j) (rate of use) of the amount of maximum radioresources permitted to the mobile station MS_(j) to the radio resourcesactually occupied by the communication is 0-60%, the evaluation point g₁(r_(j)) is “four points” (the highest score) and the evaluation point g₁(r_(j)) becomes the lower one by one like “three points”, “two points”,“one point”, and “zero point” as the usage rate r_(j) becomes thelarger. When the current traffic amount of the mobile station MS_(j) issmall (the usage rate is small), it is considered that a probability ishigh for the small amount of the radio resources being available in anadjacent radio base station, and handover of the mobile station MS_(j)to the adjacent radio base station will be easy. Further, as thetransmission speed becomes the higher (the amount of communicationsincreasing), required transmission electric power becomes the larger,and it will be difficult for the mobile station MS_(j) to communicatewith a distant radio base station. Therefore, the higher the evaluationpoint g₁ (r_(j)) is, it indicates a situation that the more it issuitable for carrying out the handover of the mobile station MS_(j) tothe adjacent station.

[0113] When the usage rate R_(i) of the radio resources in the radiobase station (partner station) BS_(i) which is performing the currentcommunication with the mobile station becomes 0-20%, the evaluationpoints g₂ (R_(i)) is “four points” (the highest score), and theevaluation point g₂ (R_(i)) becomes the higher one by one like “onepoint”, “two points”, “three points”, and “four points” as the usagerate R_(i) becomes the larger. Therefore, the higher the evaluationpoint g₂ (R_(j)) is, there is the more margin in the radio resources inthe radio base station (partner station) BS_(i) which is communicatingwith the mobile station, indicating that the combination of the mobilestation and the radio base station BS_(i) is suitable for communication.

[0114] On the other hand, if the R_(i) of a radio base station (adjacentstation) BS_(i) which is adjacent to the radio base station which iscommunicating with the mobile station also falls between 0% and 20%, theevaluation point g₂ (R_(i)) is “four points” (the highest score), andthe evaluation point g₂ (R_(i)) becomes the smaller one by one like“three points”, “two points”, “one point”, and “zero point” as the usagerate R_(i) becomes the larger. Therefore, the higher the evaluationpoint g₂ (R_(i)) is, the larger is the amount of the margin in the radioresources in the radio base station BS_(i) which is adjacent to theradio base station communicating with the mobile station, and itindicates that the combination of the mobile station and the adjacentstation BSi is in a suitable situation for communication, i.e., it isdesirable to handover the mobile station from the radio base stationcurrently communicating to the adjacent radio base station.

[0115] When the margin amount of the radio resources is small in boththe radio base station (partner station) and the radio base station(adjacent station) adjacent to the partner station serving as acommunicating counterpart of a mobile station, there is a case where themobile station may communicate by establishing links with a plurality ofradio base stations. In consideration of this type of the situation, themobile station management unit 19 evaluates combinations as mentionedabove of a mobile station and a radio base station, in addition, givesan evaluation point D about combinations of a mobile station and one ormore radio base stations to be linked. The evaluation point D is definedbased on an evaluation point table as shown in FIG. 15, for example.

[0116] As for a mobile station, it is desirable to maintain the ongoingcommunication with the radio base station as much as possible, and evenif it is the case where the communicating counterpart of the mobilestation is switched to other radio base stations, the number of theradio base stations for switching is desired to be as small as possiblefor ease of controlling and the like. From such a viewpoint, theevaluation point is defined as shown in FIG. 15.

[0117] In FIG. 15, when the mobile station maintains the communicationwith the radio base station (own station) with which a communication isongoing, that is, a handover of the mobile station will not take place,the highest evaluation point of “five points” is given. In addition, theevaluation point becomes lower as the number of the radio base stationsafter switching the communicating counterpart of a mobile stationincreases. And where the communicating counterpart for the mobilestation is switched to a plurality of radio base stations (two, three,and more than it), the evaluation point will be higher when the radiobase station (own station) which is performing the current communicationis included in the radio base stations after the switching than when itis not included.

[0118] The mobile station management unit 19 performs a process forselecting a combination of a mobile station and a radio base stationsuitable for the mobile station for newly communicating after carryingout handover based on each evaluation point as mentioned above of thecombinations of a mobile station and a radio base station. This processis performed as follows, for example.

[0119] First, an evaluation point y_(ij) about the mobile station MS_(j)communicating with the radio base station BS_(j) after handover, i.e.,the evaluation point y_(ij) for the combination of the mobile stationMS_(j) and the radio base station BS_(i), using each evaluation points f(P_(ij)), g₁ (r_(j)) and g₂ (R_(i)) mentioned above is calculatedaccording to the formula presented below.

y _(ij) =αf(P _(ij))+βg ₁(r _(j))+γg ₂(R _(i))  (1)

[0120] In the above-mentioned formula (1), α, β, and γ are weightingconstants predefined based on required conditions of a system.

[0121] Further, a final evaluation point z for the combination of themobile station MS_(j) and the radio base station _(i) is calculatedusing the above-mentioned evaluation point D (refer to FIG. 15)

Z=y _(ij) +εD  (2-1)

[0122] And, for example, as shown in FIG. 16, each evaluation valuewhich is calculated according to the above formula (2-1) is summarizedin the management table format for each of the combinations of MS2, MS3and MS4 that are communicating with the radio base station BS2 that willbecome congested, and the mobile station MS1 that has entered into thecommunication area of the radio base station BS2 and the radio basestations BS2, BS1, BS3 and so on.

[0123] Further, the evaluation point z about the combination of themobile station MS_(j) and two radio base stations BS_(i1) and BS_(i2)and the combination of the mobile station MS_(j) and three radio basestations BS_(i1), BS_(i2), and BS_(i3) are calculated according to theformula in below, respectively.

z=(y _(i1j) +y _(i2j))/2+εD  (2-2)

z=(y_(i1j) +y _(i2j) +y _(i3j))/3+εD  (2-3)

[0124] In the above-mentioned formulae (2-2) and (2-3), ε is a weightingconstant. Further, each of the above-mentioned radio base stationsBS_(i1), BS_(i2), and BS_(i3) is selected in the descending order of themagnitude of the value y_(ij) that is calculated as above to the radiobase station combined with the mobile station MS_(j).

[0125] The evaluation points z calculated as above for combinations ofeach mobile station MS_(j) and a plurality (2 or 3) of the radio basestations are added to the management table as shown in FIG. 16.

[0126] When the management table is completed as mentioned above, thecombination of the mobile station which should carry out handover andthe radio base station of the handover destination will be selected withreference to the management table.

[0127] For example, with reference to the management table shown in FIG.16, combinations of the mobile station BS_(j) and the radio base stationBS_(i) are searched one by one in the descending order of the evaluationpoint z, and the combination with the highest evaluation point, otherthan the combination of the mobile station and radio base station whichare currently communicating, is selected from the combinations. Thismeans that the most suitable combination for communication between themobile station and radio base station in the combinations other than thecombination of the mobile station and radio base station which areperforming the current communication is selected. As a result ofselecting the combination of the mobile station and radio base stationsas mentioned above, in this example, the combination of the mobilestation MS2 and the radio base stations BS2 and BS3 is selected.

[0128] When the selection of the combination of the mobile station MS2and the radio base stations BS2 and BS3 is made as mentioned above, thehandover process is executed based on directions from the RNC 10 asmentioned above (refer to FIG. 11(c)). Consequently, the communicatingcounterpart of the mobile station MS2 which was communicating with theradio base station BS2 is switched to the radio base stations BS2 andBS3 (partial handover).

[0129] Although the example mentioned above described the case where theradio resources in the radio base station BS2 will run short when themobile station MS1 moves into the communication area of the radio basestation BS2 (traffic congestion), similar process can be performed whenan amount of data communications of any of the mobile stations MS2, MS3,and MS4 which are communicating with the radio base station BS2increases or when a new mobile station start communication with theradio base station BS2, causing a shortage of the radio resources in theradio base station BS2.

[0130] Further, although the number of the combinations of the mobilestations and radio base stations which are selected was one in theexample mentioned above, it is not limited to that. A plurality ofcombinations of mobile stations and radio base stations that become thecommunicating counterpart may be selected. In this case, thecombinations are selected in the descending order of the evaluationpoint as mentioned above. The number of the combinations can be definedbased on a relationship between an available amount of the radioresources in the radio base station, and the radio resources required bya mobile station which newly starts communication (by handover or a newcall), and by a mobile station where the amount of data communicationsincreased rapidly in the communication with the radio base station.

[0131] Further, the sixth example of the handover system concerning theimplementation mode of the present invention will be described. In thesixth example, each mobile station autonomously switches the radio basestation serving as the communicating counterpart (a handover) based oncommunication conditions. For this purpose, each mobile station has thesimilar function as the mobile station management unit 19 provided inthe RNC 10 mentioned above.

[0132] In the sixth example of the handover system, a process isperformed according to a procedure as shown in FIG. 17, for example.

[0133] In FIG. 17, the radio resources management unit 12 of the RNC 10requires a report about the availability (or usage) of radio resourcesfrom each of the radio base stations BS1, BS2, and BS3 (1). Each of theradio base stations BS1, BS2, and BS3 reports the availability of theradio resources to the radio resources management unit 12 of the RNC 10,responding to the demand (1).

[0134] The radio resource management unit 12 reports the informationabout the availability of the radio resources in each of the reportedradio base stations BS1, BS2, and BS3 to each of the serving mobilestations, e.g., the mobile stations MS1 and MS2 through the radio basestations BS1 and BS2 (2).

[0135] Each of the mobile stations MS1 and MS2 measures the receivingelectric field intensity from radio base stations that are adjacent tothe radio base stations BS1 and BS2, respectively, under currentcommunication. And based on the availability of the radio resources inthe radio base stations which are adjacent to the radio base stationunder current communication notified as mentioned above, and themeasured receiving electric field intensity mentioned above, each of themobile stations MS1 and MS2 calculates the evaluation point thatindicates the degree of the suitableness of each radio base station fora communicating counterpart to the mobile station concerned, like theexample mentioned above (3). The calculation method is the same as themethod mentioned in the fifth example mentioned above (a function of themobile station management unit 19). Each of the mobile stations MS1 andMS2 further creates a management table as shown in FIG. 16 thatdescribes the evaluation points for the combinations of the mobilestation concerned and each of the radio base stations which arecalculated as mentioned above (4).

[0136] A notice to each of the mobile stations MS1 and MS2 of theinformation about the available radio resources in each above-mentionedradio base station is made at an every predetermined interval. Andwhenever each of the mobile stations MS1 and MS2 receives the notice, itupdates the evaluation point described in the management table mentionedabove based on the notified information and the measured value of thereceiving electric field intensity from each radio base station. If thecommunication traffic in a certain radio base station increases and itwill be in a traffic congestion condition, the evaluation point of amobile station which communicates with the radio base station for acombination with one or more other radio base stations becomes higherthan the evaluation point of the combination with the radio base stationunder current communication. When the difference of those evaluationpoints exceeds the predetermined standard, the one or more radio basestations concerned are selected as the communicating counterpart of themobile station concerned (3).

[0137] Then, the mobile station autonomously switches itself to theradio base station of the selected combination as the communicatingcounterpart as mentioned above (handover) (4). Henceforth, the mobilestation communicates with other terminals through the switched radiobase station, the RNC 10, and the backbone.

[0138] In such a handover system as above, each mobile station managesthe evaluation about the combination with the radio base station undercurrent communication, and other combinations with one or more radiobase stations, and each mobile station switches its communicatingcounterpart autonomously based on the situation managed. Accordingly,the RNC 10 does riot have to manage the evaluation point of thecombinations of each serving mobile station and radio base stationscentrally, and the process load of the RNC 10 is mitigated.

[0139] Further, the seventh example of the handover system concerningthe implementation mode of the present invention will be described. Inthe seventh example, each mobile station switches the radio base stationas its communicating counterpart (handover) by an autonomous controlbased on communication conditions, similarly to the sixth examplementioned above. In this example, each mobile station chooses a radiobase station as a new communicating counterpart without making amanagement table (referring to FIG. 16) as mentioned above.

[0140] Here a case is assumed, for example, where communication data ofa mobile station which communicates with a certain radio base stationincreased rapidly, and the radio base station concerned turns into atraffic congestion condition.

[0141] Each mobile station processes according to the procedure shown inFIG. 18.

[0142] For example, the case where the communication data of the mobilestation MS1 which communicates with the radio base station BS1 increasesrapidly, and the radio base station BS1 concerned is in trafficcongestion is assumed. In this case, the mobile station MS1 compares anamount of the radio resources Ru which is newly needed for the increasedamount of communication data with an amount of the available radioresources R1 in the radio base station BS1 reported from the RNC 10 asmentioned above (S1). If the result of the comparison is that the amountof the available radio resources R1 in the radio base station BS1 islarger than the amount of the radio resources Ru that is newly needed asmentioned above (YES in S1), the mobile station MS1 will maintain thecommunication condition with the radio base station BS1 (S2), that is,the handover of the mobile station MS1 is not made in this case.

[0143] On the other hand, it the amount R1 of the available radioresources in the above-mentioned radio base station BS1 is smaller thanthe radio resources amount Ru that is newly needed for the datacommunications by the mobile station MS1 as mentioned above (itrepresents NO in S1), the mobile station MS1 measures the receivingelectric field intensity from the radio base stations BS2 and BS3 thatare adjacent to the radio base station BS1 (S3). In addition, priorityof the adjacent radio base stations is decided according to themagnitude of the receiving electric field intensity.

[0144] The mobile station MS1 first determines whether the electricfield intensity from a radio base station, BS2 for example, that ranksthe highest priority (providing the highest receiving electric fieldintensity) is sufficient for communication (S4). If the electric fieldintensity is not sufficient for communication (it is NG in S4), themobile station MS1 will maintain the communication with the radio basestation BS1 (handover suspended). In this case, the radio base stationBS1 assigns radio resources to the communication for the mobile stationMS1 within the limit of the available radio resource R1.

[0145] On the other hand, if the electric field intensity from the radiobase station BS2 is enough for communication (OK in S4), the mobilestation MS1 will determine further whether an amount R2 of the availableradio resources of the radio base station BS2 reported from the RNC 10as mentioned above is larger than the amount Ru of the radio resourcesthat is newly needed by the mobile station MS1 concerned (S5). If theamount R2 of available radio resources of this radio base station BS2 islarger the amount Ru of the radio resources newly needed by the mobilestation MS1 (YES in S5), the mobile station MS1 will nominate the radiobase station BS2 as a communicating counterpart (S6). The communicatingcounterpart of the mobile station MS1 is switched from the radio basestation BS1 to the radio base station BS2 by the switching control ofthe RNC 10 based on the nomination (handover is carried out).

[0146] Further, if the amount R2 of available radio resources of theradio base station BS2 is smaller than the amount Ru of radio resourceswhich is newly needed by the mobile station MS1 (NO in S5), the mobilestation MS1 determines whether the total amount of available radioresources R1 and R2 of the radio base stations BS1 and BS2,respectively, (R1+R2) is larger than the amount Ru of [the radio]unrelated resources that is newly needed by the mobile station MS1concerned (S7). If the total amount (R1+R2) is larger than the amount Ruof radio resources newly needed by the mobile station MS1 concerned (YESin S7), the mobile station MS1 nominates the two radio base stations BS1and BS2 as the communicating counterparts (S8). The communicatingcounterpart of the mobile station MS1 is switched from one station ofthe radio base station BS1 to two stations of the radio base stationsBS1 and BS2 by the switching control of the RNC 10 based on thenomination (partial handover is carried out).

[0147] On the other hand, if the total of the amounts R1 and R2 (R1+R2)of the available radio resources of the both above-mentioned radio basestations BS1 and BS2 is smaller than the amount Ru of radio resourcesnewly needed by the mobile station MS1 concerned (NO in S7), the mobilestation MS1 determines whether the electric field intensity from theradio base station BS3, for example, where the priority is high(receiving electric field intensity is high), having given the highestelectric field intensity next to the radio base station BS2, givessufficient electric field intensity for communications (S9). If theelectric field intensity is not enough for communications (NG in S9),the mobile station MS1 will maintain communication with the radio basestation BS1 like an above-mentioned case (NG in S4) (handoversuspended).

[0148] If the electric field intensity from the above-mentioned radiobase station BS3 is enough for communications with the mobile stationMS1 (OK in S9), the mobile station MS1 concerned will determine furtherwhether the amount R3 of available radio resources in the radio basestation BS3 reported from the RNC 10 as mentioned above is larger thanthe amount Ru of radio resources newly needed (S10). If the amount R3 ofthe available radio resources in the radio base station R3 is largerthan the amount Ru of radio resources newly needed by the mobile stationMS1 (YES in S10), the mobile station MS1 nominates the radio basestation BS3 as the communicating counterpart (S11). The communicatingcounterpart of the mobile station MS1 is switched from the radio basestation BS1 to the radio base station BS3 by the switching control ofthe RNC 10 based on this nomination (handover is carried out).

[0149] If, on the other hand, the amount R3 of the available radioresources of the radio base station BS3 is smaller than the newly neededamount Ru by the mobile station MS1 (NO in S10), the mobile station MS1determines whether the total amount of R1 and R2 of the available radioresources of the radio base stations BS1 and BS3 (R1+R3) is larger thanthe amount Ru of [the radio] unrelated resources newly needed by themobile station MS1 concerned (S12). If the total amount (R1+R3) islarger than the amount Ru of radio resources newly needed by the mobilestation MS1 concerned (YES in S12), the mobile station MS1 nominates thetwo radio base stations BS1 and BS3 as the communicating counterpart(S13). The communicating counterpart of the mobile station MS1 isswitched from one station of the radio base station BS1 to two stationsof the radio base stations BS1 and BS3 by the switching control of theRNC 10 based on this nomination (partial handover is carried out).

[0150] If the total amount of the amounts R1 and R3 of the availableradio resources of the both of the above-mentioned radio base stationsBS1 and BS3 (R1+R3) is smaller than the amount Ru of radio resourceswhich is newly needed by the mobile station MS1 concerned (NO in S12),the mobile station MS1 determines whether a total of the amounts R2 andR3 of available radio resources of the two radio base stations BS2 andBS3 adjacent to the radio base station BS1 (R2+R3) is larger than theamount Ru of radio resources newly needed by the mobile station MS1concerned (S14). If the total amount (R2+R3) is larger than the amountRu of radio resources that is newly needed as mentioned above (YES inS14), the mobile station MS1 will nominate the two radio base stationsBS2 and BS3 as the communicating counterpart (S15). The communicatingcounterpart of the mobile station MS1 is switched to the two stations-ofthe radio base stations BS2 and BS3 from the radio base station BS1 bythe switching control of the RNC 10 based on this nomination (handoverto two or more stations).

[0151] Further, when the total amount R2 and R3 (R2+R3) of the availableradio resources of the both radio base stations BS2 and BS3 mentionedabove is smaller than the amount Ru of radio resources newly needed bythe mobile station MS1 concerned (NO in S14), the mobile station MS1determines whether a total amount of radio resources R1, R2, and R3 ofthree radio base stations BS1, BS2, and BS3 (R1+R2+R3) is larger thanthe amount Ru of radio resources which is newly needed by the mobilestation MS1 concerned (S16). If the total amount (R1+R2+R3) is largerthan the amount Ru of radio resources newly needed by the mobile stationMS1 concerned (YES in S16), the mobile station MS1 will nominate thethree radio base stations BS1, BS2, and BS3 as the communicatingcounterpart (S17). The communicating counterpart of the mobile stationMS1 is switched from one station of the radio base station BS1 to threestations of the radio base stations BS1, BS2, and BS3 by the switchingcontrol of the RNC 10 based on this specification (partial handover).

[0152] If, on the other hand, there are no other adjacent radio basestations when the total amount of the radio resources (R1+R2+R3)mentioned above is smaller than the amount Ru of radio resources newlyneeded by the mobile station MS1 concerned (NO in S16), the mobilestation MS1 will maintain communication with the radio base station BS1like the case (NG in S4 and NG in S9) mentioned above (handoversuspended).

[0153] The above process is performed by the mobile station when theamount of data communications of the mobile station which iscommunicating with the radio base station increases rapidly. Therefore,when a mobile station under communication enters into the communicationarea of a certain radio base station, the radio base station concernedmay turn into a traffic congestion condition, then it is uncertain whichmobile station that is communicating with the radio base stationconcerned should execute the process relative to the handover accordingto the procedure mentioned above (refer to FIG. 18). Then, by providingthe mobile station management unit 19 in the RNC 10 as mentioned above,a suitable mobile station can switch radio base stations for thecommunicating counterpart by autonomous control based on communicationsituations in the case mentioned above.

[0154] For example, when a mobile station advances into thecommunication area, all the mobile stations that are already incommunication with the radio base station which will be in a trafficcongestion condition measure the receiving electric field intensity fromthe radio base station concerned, and each mobile station reports themeasured value to the mobile station management unit 19 of the RNC 10.The mobile station management unit 19 gives a priority in an ascendingorder of the reported low receiving electric field intensity of eachmobile station. In addition, the mobile station management unit 19directs to perform the handover process to the mobile station of thehighest priority (receiving electric field intensity being the lowest).The directed mobile station executes the handover process according tothe procedure as shown in FIG. 18. The radio resources amount Ru isequal to the amount of radio resources which the specified mobilestation is presently using in this case.

[0155] If the amount of the radio resources made available by thehandover of the mobile station mentioned above is not large enough toreach the amount of radio resources which is to be used by a mobilestation which advances into the communication area, or the handover ofthe mobile station specified to carry out the handover as mentionedabove was not possible, an handover direction is issued to a mobilestation of the next priority (the receiving electric field intensitybeing the second lowest) to carry out the handover process according tothe procedure shown in FIG. 18. In addition, the process for nominatinga mobile station and executing the handover at the nominated mobilestation is repeatedly performed until the amount of radio resources madeavailable reaches the amount of radio resource to be used by the mobilestation that advances into the communication area.

[0156] In the following, the eighth example of the handover systemrelative to the implementation mode of the present invention will bedescribed.

[0157] In this example, verification is performed as to whether thecommunication situation at the mobile station which carried out thehandover is suitable after the handover of the mobile station is carriedout according to each example mentioned above.

[0158] The handover system concerning this example is structured asshown in FIG. 19.

[0159] In FIG. 19, the RNC 10 includes the electric field intensitymeasurement control unit 11, the radio resources management unit 12, theswitch 13, and the mobile station management unit 19 like the fifthexample mentioned above (refer to FIG. 10). The RNC 10, further,includes a BER (Bit Error Rate) measuring control unit 20. Each of themobile stations MS1, MS2, and MS3 is equipped with a BER measuringinstrument for measuring an error rate (BER) of an error incommunication with a radio base station.

[0160] In this example, process is performed according to the procedureshown in FIG. 20.

[0161] If the radio base station BS2 becomes in a traffic congestioncondition by the handover of the mobile station MS1 which has beencommunicating with the radio base station BS1, handover of the mobilestation MS2 which was communicating with the radio base station BS2which becomes in the traffic congestion condition will be carried outaccording to procedures similar to the procedures (a), (b), and (c)shown in FIG. 11. Consequently, the communicating counterpart of themobile station MS2 is switched from one station of the radio basestation BS2 to two stations of the radio base stations BS2 and BS3.

[0162] If the communicating counterpart of the mobile station MS2 isswitched to the radio base stations BS2 and BS3 in this manner, theerror rate in the communication of this mobile station MS2 will bechecked (refer to (d) of FIG. 20). That is, the BER measuring controlunit 20 of the RNC 10 directs the mobile station MS2 via each of theradio base stations BS2 and BS3 to measure the error rate. If the mobilestation MS2 receives the direction, the BER measuring instrument willmeasure the error rate in communication with each of the radio basestations BS2 and BS3 according to the direction. In addition, the mobilestation MS2 reports the measured error rate to the BER measuring controlunit 20 of the RNC 10.

[0163] The BER measuring control unit 20 will determine whether theerror rate is below a standard value when the error rate report isreceived from the mobile station MS2. If the error rate is below thestandard value, a notice that the handover was carried out successfullyis given to the mobile station MS2 from the BER measuring control unit20 via the radio base stations BS2 and BS3. On the other hand, if theerror rate exceeds the fixed value, the BER measuring control unit 20directs the mobile station management unit 19 to start the procedure forselecting a combination of a mobile station for handover and the radiobase station which serves as the communicating counterpart, noting thatthe combination of the mobile station MS1 and the radio base stationwhich serves as the communicating counterpart is unsuitable. Then,selection of a combination of the mobile station which should carry outhandover and the radio base station as the handover destinationaccording to the procedure (refer to FIG. 12) mentioned above is madeagain under the control of the mobile station management unit 19. Inaddition, the handover is carried out for the selected mobile station tothe radio base station that is paired therewith.

[0164] When the handover process of a mobile station which wascommunicating with the radio base station BS2 to an adjacent radio basestation is performed and a check result of the communication after thehandover is proper as described above, the handover of the mobilestation MS1 from the radio base station BS1 to the radio base stationBS2 resumes (refer to (e) of FIG. 20)

[0165] As described above, according to the present invention, thehandover of a mobile station can be carried out to other radio basestations where traffic is not congested when a certain radio basestation is experiencing a traffic congestion, thereby enabling toprovide an effective use of radio resources in the whole mobilecommunications network. Further, since the minimum data-transmissioncapacity or the minimum throughput of a user request can be secured, itbecomes possible to enhance user conveniences.

What is claimed is
 1. A handover control method that switches a radiobase station serving as a communicating counterpart of a mobile stationcomprising: detecting whether any of mobile stations communicating withthe radio base station become incapable of communicating while apredetermined minimum bandwidth secured; and switching the communicatingcounterpart of the mobile station that communicates with said radio basestation from said radio base station to another radio base station.
 2. Ahandover control method that switches the radio base station serving asthe communicating counterpart of the mobile station, comprising:detecting whether any of mobile stations communicating with the radiobase station become incapable of communicating while the predeterminedminimum bandwidth secured; and switching the communicating counterpartof the mobile station that communicates with said radio base stationfrom said radio base station to a plurality of other radio basestations.
 3. The handover control method as claimed in claim 1 or 2,wherein a radio base station whose electric field intensity was thestrongest and a mobile station that measured it are selected as themobile station as the object of the handover and the radio base stationserving as the communicating counterpart thereof, excepting a set of themobile station and the radio base station under current communication,based on electric field intensity information about the radio basestations obtained from each mobile station.
 4. The handover controlmethod as claimed in claim 1 or 2, wherein a mobile station to whichallocation of radio resources is the nearest to the minimum bandwidthand a radio base station which has the most radio resources availableare selected as the mobile station and the radio base station for thehandover object.
 5. The handover control method as claimed in claim 1 or2, wherein a mobile station that requires the radio resources in thehighest value of the minimum bandwidth and a radio base station in whichthe radio resources are available the most are selected as the mobilestation and the radio base station for the handover object.
 6. Thehandover control methods as claimed in any one of claims 1 through 5,wherein the detection of an inability to communicate while thepredetermined minimum bandwidth secured at any of mobile stations thatcommunicate with the radio base station is realized by measuring anelectric field intensity, a bit error rate, a frame error rate, a packeterror rate, a packet discarding rate, or any combination thereof andbasing on the measuring result thereof.
 7. The handover control methodsas claimed in any one of claims 1 through 6, wherein said mobile stationselects a radio base station that serves as the communicatingcounterpart after said switching.
 8. The handover control methods asclaimed in any one of claims 1 through 6, wherein the radio base stationwhich serves as the communicating counterpart after said switching isselected by a node other than said mobile station; informationindicative of the selected radio base station is reported to said mobilestation from said node; and the radio base station which serves as thecommunicating counterpart of said mobile station is switched to thereported radio base station.
 9. The handover control methods as claimedin any one of claims 1 through 8, comprising detecting when acommunication securing a predetermined minimum bandwidth becomesimpossible at any of mobile stations that are communicating with a radiobase station in a first radio communication system that employs a firstcommunication protocol; and transferring information relative to radioresources of said mobile station from said first radio communicationsystem to said second radio communication system via a wired sectionupon converting the protocol, when switching the communicatingcounterpart of the mobile station that communicates with said radio basestation to the second radio base station that employs the secondcommunication protocol when said detection was made.
 10. A handovercontrol method which switches a radio base station which serves as thecommunicating counterpart of a mobile station, comprising: deciding asto whether communication of a radio base station will be in a trafficcongestion condition; selecting a combination of a mobile station thatcommunicates with said radio base station and one or more radio basestations with which said mobile station can communicate according to apredetermined standard when the decision is made that the communicationof the radio base station will become congested; and switching thecommunicating counterpart of the mobile station in the selectedcombination to one or more radio base stations in the combination. 11.The handover control method as claimed in claim 10, wherein thepredetermined standard for selecting a combination of said mobilestation and a radio base station is defined based upon an availableamount of radio resources of a radio base station or an amount of theradio resources required.
 12. The handover control methods as claimed inclaim 10 or 11, wherein the predetermined standard for selecting thecombination of said mobile station and a radio base station is definedbased upon receiving electric field intensity of the communicationbetween the mobile station and the radio base station.
 13. The handovercontrol methods as claimed in any one of claims 10 through 12, whereinthe predetermined standard for selecting the combination of said mobilestation and a radio base station is defined based upon the ratio of theamount of radio resources permitted to the mobile station to the amountof radio resources currently used in fact.
 14. The handover controlmethods claimed in any one of claims 10 through 13, wherein thepredetermined standard for selecting the combination of said mobilestation and a radio base station is defined based upon the number of theradio base stations which should perform simultaneous communicationsafter switching the communicating counterpart.
 15. The handover controlmethods as claimed in any one of claims 10 through 14, wherein thepredetermined standard for selecting the combination of said mobilestation and a radio base station is defined based upon whether the radiobase station which is performing the current communication is includedin the radio base stations with which the mobile station is tocommunicate after switching the communicating counterpart.
 16. Thehandover control method as claimed in any one of claims 10 through 15,wherein a priority about the appropriateness of mutual communication isgiven to combinations of each mobile station that communicates with saidradio base station and said radio base station and one or more of radiobase stations adjacent to the radio base station in accordance with saidpredetermined standard; and a selection is made in favor of thecombination of a mobile station and a radio base station with a higherpriority.
 17. The handover control methods as claimed in any one ofclaims 10 through 16, wherein the process for selecting a combination ofany of mobile stations that communicate with said radio base station andone or more radio base stations that can communicate with said mobilestation is performed by a node that is connected to each radio basestation via a wired section.
 18. The handover control methods as claimedin any one of claims 10 through 16, wherein a mobile station thatcommunicates with said radio base station performs the process forselecting a combination of said mobile station and one or more radiobase stations with which said mobile station can communicate accordingto said predetermined standard.
 19. The handover control method asclaimed in claim 18, wherein said mobile station which communicates withsaid radio base station determines whether said radio base station is ina traffic congestion condition.
 20. The handover control methods asclaimed in any one of claims 1 through 19, wherein the communicationconditions between the mobile station and one or more radio basestations are supervised after switching the communicating counterpart ofsaid mobile station to said one or more radio base stations; and aprocess is performed such that the communicating counterpart of saidmobile station may be switched again when the supervised communicationcondition turned into a condition poorer than the predetermined standardcondition.
 21. The handover control method as claimed in claim 20,wherein an error rate in communication between a mobile station and oneor more radio base stations is supervised as said communicationcondition.
 22. A handover system that switches radio base stations asthe communicating counterpart of a mobile station, comprising: detectionmeans for detecting an inability of any of mobile stations that arecommunicating with a radio base station to communicate while apredetermined minimum bandwidth condition secured; and switching controlmeans for switching the communicating counterpart of the mobile stationcommunicating with said radio base station from said radio base stationto another base station when said detection means detected that anymobile station is unable to communicate while the predetermined minimumbandwidth condition secured.
 23. A handover control system that switchesradio base stations as the communicating counterpart of a mobilestation, comprising: detection means for detecting an inability of anyof mobile stations that are communicating with a radio base station tocommunicate while a predetermined minimum bandwidth condition secured;and switching control means for switching the communicating counterpartof the mobile station communicating with said radio base station fromsaid radio base station to a plurality of other base stations when saiddetection means detected that any mobile station is unable tocommunicate while the predetermined minimum bandwidth condition secured.24. The handover control system as claimed in claim 22 or 23, whereinsaid switching control means comprises selection means for selecting aradio base station whose electric field intensity is the strongest andthe mobile station that measured it as the mobile station and the radiobase station serving as the communicating counterpart for the handover,based on the electric field intensity information relative to radio basestations obtained from each mobile station, excepting the combination ofthe mobile station and the radio base station in current communication.25. The handover control system as claimed in claim 22 or 23, whereinsaid switching control means comprises selection means for selecting acombination of a mobile station that is assigned an amount of radioresources the closest to the minimum bandwidth and a radio base stationwhere the radio resources are available the most as the mobile stationand the radio base station for the handover.
 26. The handover controlsystem as claimed in claim 22 or 23, wherein said switching controlmeans comprises selection means for selecting a combination of a mobilestation that requires a large amount of the radio resource with thehighest value of the minimum bandwidth and a radio base station wherethe radio resources are available the most as the mobile station and theradio base station for the handover.
 27. The handover control system asclaimed in any one of claims 22 through 26, wherein said detection meanscomprises measurement means for measuring an electric field intensity,bit error rate, a frame error rate or a packet discarding rate, or anycombinations thereof, and the detection of the inability to communicateof any mobile stations that are communicating with the radio basestation while the predetermined minimum bandwidth secured, based onmeasurement results by said measurement means.
 28. The handover controlsystem as claimed in any one of claims 22 through 27, wherein saidmobile station selects the radio base station serving as thecommunicating counterpart after said switching.
 29. The handover controlsystem as claimed in any one of claims 22 through 27, wherein, the radiobase station which serves as the communicating counterpart after saidswitching is selected by a node other than said mobile station; theinformation on the selected radio base station is reported to saidmobile station from said node; and the switching is made to the reportedradio base station as the base station to serve as the communicatingcounterpart of said mobile station.
 30. The handover control system asclaimed in any one of claims 22 through 29, wherein said detection meansdetects that any mobile station among mobile stations undercommunication with a radio base station of the first radio communicationsystem that employs the first communication protocol becomes unable tocommunicate in the condition that the predetermined minimum bandwidth issecured, said handover control system comprising means that transfersinformation about radio resources of said mobile station from said firstradio communication system to said second radio communication systemupon protocol conversion when the communicating counterpart of themobile station that communicates with said radio base station isswitched to a radio base station in the second radio communicationsystem that employs the second communication protocol by said switchingcontrol means when said detection means makes said detection.
 31. Inhandover control systems that switch a radio base station which servesas communicating counterpart of a mobile station, comprising: trafficcongestion checking means for deciding whether communication of a radiobase station will be in a traffic congestion condition; selection meansfor selecting a combination of any mobile station that is communicatingwith said radio base station and one of a plurality of radio stationswith which communication is possible with said mobile station inaccordance with a predetermined standard when said traffic congestionchecking means determines that the radio base station will be in atraffic congestion condition; and switching control means for switchingthe communicating counterpart of the mobile station in the selectedcombination to one or more radio base stations in the combination. 32.The handover control system as claimed in claim 31, wherein thepredetermined standard for selecting a combination of said mobilestation and radio base station is defined based on the amount ofavailable radio resources in the radio base station, or the amount ofradio resources required.
 33. The handover control system as claimed inclaim 31 or 32, wherein the predetermined standard for selecting acombination of said mobile station and a radio base station is definedbased upon the receiving electric field intensity in communicationbetween mobile stations and radio base stations.
 34. The handovercontrol system as claimed in claim 31 or 33, wherein the predeterminedstandard for selecting a combination of said mobile station and a radiobase station is defined based upon the ratio of the amount of radioresources permitted to the mobile station to the amount of radioresources currently used in fact.
 35. The handover control system asclaimed in any one of claims 31 through 34, wherein the predeterminedstandard for selecting a combination of said mobile station and a radiobase station is defined based upon the number of the radio base stationswhich should perform simultaneous communication after switching thecommunicating counterpart.
 36. The handover control system as claimed inany one of claims 31 through 35, wherein the predetermined standard forselecting a combination of said mobile station and a radio base stationis defined based upon whether the radio base station which is performingthe current communication is included.
 37. The handover control systemas claimed in any one of claims 31 through 36, wherein, said selectionmeans gives the priority about the appropriateness of mutualcommunication to combinations of each mobile station that communicateswith said radio base station and said radio base station and one or moreadjacent radio base stations; and a combination of a mobile station anda radio base station which is given with the highest priority isselected.
 38. The handover control system as claimed in any one ofclaims 31 through 37, wherein said selection means is provided in a nodeconnected to each radio base station via a wired section.
 39. Thehandover control system as claimed in any one of claims 31 through 37,wherein a mobile station which communicates with said radio base stationselects a combination of said mobile station and one or more radio basestations with which communication is possible in accordance with saidpredetermined standard.
 40. The handover control system as claimed inclaim 39, wherein said mobile station that communicates with said radiobase station comprises said traffic congestion detection means.
 41. Thehandover control system as claimed in any one of claims 22 through 40,comprising: communication condition supervision means for supervisingthe communication condition between the mobile station and one or moreradio base stations after switching the communicating counterpart ofsaid mobile station to said one or more radio base stations; conditiondecision means for deciding whether the communication conditionsupervised by said communication condition supervision means will be ina condition poorer than the predetermined standard condition; andre-switching control means for performing process for switching thecommunicating counterpart of said mobile station again when saidcondition decision means determines that the communication condition assupervised is in a condition poorer than the predetermined standardcondition.
 42. The handover control system as claimed in claim 41,wherein said communication condition supervising means comprises errorrate detection means for supervising an error rate in communicationbetween a mobile station and one or more radio base stations as saidcommunication condition.